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Wettability of Sc-CO2/Water/Quartz Systems: Simultaneous Measurement of Contact Angle and Interfacial Tension at Reservoir Conditions. soheil saraji, Lamia Goual, Mohammad Piri, and Henry Plancher Langmuir, Just Accepted Manuscript • DOI: 10.1021/la3050863 • Publication Date (Web): 30 Apr 2013 Downloaded from http://pubs.acs.org on May 1, 2013
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Wettability of Sc-CO2/Water/Quartz Systems: Simultaneous Measurement of Contact Angle and Interfacial Tension at Reservoir Conditions Soheil Saraji, Lamia Goual*, Mohammad Piri, and Henry Plancher Department of Chemical and Petroleum Engineering, University of Wyoming 1000 E. University Avenue, Laramie, WY 82071-2000, USA
KEYWORDS Interfacial tension, Contact angle, Supercritical CO2, Wettability, Density, ADSA-NA
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ABSTRACT
Injection of carbon dioxide in deep saline aquifers is considered as a method of carbon sequestration. The efficiency of this process is dependent on the fluid-fluid and rock-fluid interactions inside the porous media. For instance, the final storage capacity and total amount of capillary-trapped CO2 inside an aquifer are affected by the interfacial tension between the fluids and the contact angle between the fluids and the rock mineral surface. A thorough study of these parameters and their variations with temperature and pressure will provide a better understanding of the carbon sequestration process and thus improve predictions of the sequestration efficiency. In this study, the controversial concept of wettability alteration of quartz surfaces in the presence of supercritical carbon dioxide (sc-CO2) was investigated. A novel apparatus for measuring interfacial tension and contact angle at high temperatures and pressures based on Axisymmetric Drop Shape Analysis with no-Apex (ADSA-NA) method was developed and validated with a simple system. Densities, interfacial tensions, and dynamic contact angles of CO2/water/quartz systems were determined for a wide range of pressures and temperatures relevant to geological sequestration of CO2 in the subcritical and supercritical states. Image analysis was performed with an advanced Axisymmetric Drop Shape Analysis with no-Apex (ADSA-NA) method that allows the determination of both interfacial tensions and contact angles with high accuracy. The results show that supercritical CO2 alters the wettability of quartz surface towards less water-wet conditions compared to subcritical CO2. Also we observed an increase in the water advancing contact angles with increasing temperature indicating less water-wet quartz surfaces at higher temperatures.
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1. INTRODUCTION The safe trapping of carbon dioxide in deep saline aquifers is considered as a viable mean for controlling carbon emission to the atmosphere.1 The main trapping mechanisms of CO2 include: structural/stratigraphic trapping, residual capillary trapping, solubility trapping, mineral trapping, and local capillary trapping.2 Successful implementation of CO2 sequestration projects via the structural trapping mechanism requires minimal CO2 leakage through the cap rock. One possible cause of rapid leakage is capillary failure of the cap rock, which is to a large extent controlled by the CO2/brine interfacial tension (IFT) and CO2/brine/cap rock mineral contact angle (CA).3 Rapid leakage occurs when the pressure build-up in the reservoir due to CO2 injection is more than the threshold value. If the wettability of the cap rock changes during CO2 sequestration from strongly water-wet to weakly water-wet, the capillary threshold pressure reduces. This will affect the design and planning of any CO2 injection and storage project as it sets a limit on both the injection pressure and the storage capacity.3 On the other hand, capillary trapping of carbon dioxide is desired for higher storage efficiencies and reduced risk of leakage through the cap rock. Capillary trapping is governed by relative permeability and capillary pressure of the rock, which in turn depend on CO2/brine IFT and wettability (or contact angle) of the reservoir rock minerals. Thus carefully measured IFT and CA data that are consistent with reservoir pressure, temperature, and fluid composition will reduce the uncertainties associated with the prediction of flow properties that directly affect the estimation of storage capacity and the fate of the CO2 plume. The wettability of minerals in the presence of water and carbon dioxide has become a controversial issue in recent years. While some studies4-6 reported no effect of CO2 phase change (or pressure) on the wettability of minerals, others7-14 found a gradual or sudden wettability 3
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change towards less water-wet conditions as pressure increased. Espinoza and Santamarina4 conducted sessile drop experiments in the pressure range of 0-1500 psi at ambient temperature and found that contact angles are independent of pressure. Bikkina5 used the same method and did not observe any clear trends with pressure (200-3000 psi) or temperature ranging from 25 to 50 °C. Dickson et al.7, Yang et al.8, Sutjiadi-Sia et al.9, and Wang et al10 reported an increase in contact angles with increasing pressure. Plung and Bruining11 and Plung et al.12 performed capillary pressure measurements on sand and coal samples. They noticed that the wettability of these samples was altered from water-wet to intermediate-wet in the presence of CO2 under supercritical conditions. Recently, Jung and Wan13 observed about 18° increase in the static contact angles from subcritical to supercritical pressures. The increase occurred only in a small pressure range close to the phase change region of CO2. Contact angles were almost constant at lower and higher pressures. Chiquet et al.3 and Broseta et al.2 used a different approach and measured both advancing and receding contact angles in a quasi-static mode. In a first study, the authors found a gradual increase in the receding contact angle with pressure.3 In a subsequent study, they reported very small variations in the receding angles while the water advancing angles showed a clear increase trend with pressure.2 Kim et al.14 studied the dewetting of silica surfaces using a water-wet glass micromodel. They observed thinning of the water film and growth of the water droplets during drainage of the initially water-saturated model with carbon dioxide. They also noticed an increase in the apparent contact angles up to 80 degrees. These findings contradict the study performed by Chalbaud and co-workers.6 The discrepancies observed in the literature may stem from differences in cleaning procedures, equilibration states of the phases, and measurement conditions and methods. Potential sources of contamination at interfaces are fluids (water and carbon dioxide), transfer lines, connections, 4
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cells, and substrate surfaces. Impurities and dissolution effects from metallic surfaces can also contribute to contamination of the system and affect contact angle values thereby causing misinterpretation of the data, as mentioned by Mahandavan.15 Moreover, there are various procedures used for cleaning substrates in the literature ranging from rinsing with weak solvents5,13,14 to extensive washing with acid solutions3. Another important factor is the preequilibration of partially miscible fluids prior to each measurement. There are different criteria and time periods for equilibration used in the literature that range from an hour to about a day.2,4 Also, observed trends of contact angle versus pressure can be affected by the state of the CO2 phase. For instance, some studies investigated the effect of pressure at ambient temperature4,7 while others performed measurements at elevated pressures and temperatures2,3,5,8-10,13. Since the critical temperature of carbon dioxide is about 31°C, these two groups of studies investigated two different systems at high pressures (i.e., liquid and supercritical CO2). In addition to the state of CO2, the type of angles to be measured is also important: static angles4,5,7-9,13,14 or advancing and receding angles.2,3 Neumann et al.16 emphasized that static contact angles can have any values between advancing and receding angles and therefore are not a characteristic value for the fluid-solid surface. Furthermore, there are other factors that can affect contact angle measurements such as temperature variations during the measurements and subjectivity in data analysis. In this study, the controversial concept of wettability alteration of quartz surfaces in the presence of supercritical CO2 is systematically investigated at elevated temperatures and pressures. Wettability is assessed by measuring apparent advancing and receding contact angles. The apparent contact angle is defined as the angle of the macroscopic meniscus when extrapolated to zero thickness of the thin film.17,18 A new experimental setup is developed, 5
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validated, and used to study the variations of contact angles in CO2/water/quartz systems with changes in temperature and pressure relevant to geological sequestration of CO2 in the subcritical and supercritical states. A new analysis method, Axisymmetric Drop Shape Analysis with No Apex (ADSA-NA) is used to simultaneously calculate IFT and CA.19 The program can accurately calculate IFT and CA from a single sessile drop or captive bubble image, making the measurements fast and reliable.19,20 We first validated the set-up with a simple system (decane/air/Teflon). Once validated, the apparatus was used to systematically measure the IFT and dynamic CA of equilibrated fluids in CO2/water/quartz systems under various stable pressure (± 0.5 psi) and temperature (± 0.05 °C) conditions. To the best of our knowledge, this is the first time that simultaneous measurements of interfacial tension and dynamic contact angle are performed in CO2/water/mineral systems and clear trends are reported between wettability and pressure and temperature.
2. MATERIALS AND METHODS 2.1 Materials Materials include distilled decane (HPLC grade, Sigma-Aldrich), isopropyl alcohol (HPLC grade, Sigma-Aldrich), sulfuric acid (95-98%, ACS grade, Sigma-Aldrich), Teflon AF 1600 solution (DuPont), silicon wafers (Silicon sense, Inc.), SuperSmoothtTM quartz slides (SPI supplies) with surface roughness smaller than 0.5 nm, NOCROMIX (Godax Laboratories, Inc.), nitrogen (instrumental grade, Praxair, Inc.), and CO2 (instrumental grade, Praxair, Inc.). The CO2 gas cylinders were equipped with a Syphon tube for liquid withdrawal. The distilled water was freshly obtained from an in-house all-glass water distiller.
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2.2 Experimental Setup A state-of-the-art IFT/CA measurement system was designed and constructed to perform measurements under extreme conditions such as those encountered in geological sequestration of carbon dioxide in deep saline aquifers. The setup can handle high pressure (up to 7,500 psi), high temperature (up to 200 °C), and highly corrosive fluids. All the wetted parts in the high pressure/high temperature (HPHT) setup are constructed from Hastelloy C-276, PEEK, and Sapphire glass. A few parts (valves and some fittings) are made from 316 stainless steel. The main components of the apparatus are: HPHT measurement cell, an imaging module, a fluid equilibration module, a density measurement module, a temperature control module, and a data acquisition and analysis module. Figure 1 illustrates a schematic of the entire experimental setup. 2.2.1 IFT/CA measurement cell - The cell is made of Hastelloy and is considered as the core of the experimental setup. A schematic of the interior of the cell is also provided in Figure 1. The crystal holder located inside the measurement cell contains a movable tray that holds the crystal and an opening to observe the level of the fluids inside the cell. Two drive shafts mounted on the cell facilitate the measurement of CA and IFT. The shafts move the needle and crystal tray in the vertical and horizontal directions, respectively. Two high-pressure Sapphire windows are located at two ends of the measurement cell for observing drop/bubble images generated during the measurements. 2.2.2 Imaging module – This module has two components: a light source and a microscope attached to a digital camera. The light source (Edmund Optics Inc.) consists of a blue LED, a telecentric backlight illuminator that increases the edge contrast and measurement accuracy by decreasing diffuse reflections from the object under inspection, and a polarizing filter that directs 7
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blue light in the range of 400-700 nm wavelength into the cell. The microscope has a fully apochromatically corrected lens (Leica Z16 APO) that minimizes the chromatic aberration and color fringing and therefore results in high-contrast edges and high-resolution image. The microscope is attached to a high-resolution CCD digital color camera (Leica DFC295) with a cmount interface. It offers a standard resolution of 2048 x 1536 pixels (3 Megapixels) and produces high-quality color images. These components are mounted to an adjustable stand that pivots to provide independent, adjustable vertical angles for two arms holding the microscope/camera and the light source. The measurement cell and the imaging module are fastened on an anti-vibration air table to minimize image distortion due to the mechanical noises from the environment. 2.2.3 Fluid equilibration module - A mechanical convection Yamato oven was modified to accommodate a dual-cylinder pulse-free Quizix pump and a fluid equilibration cell. The pump, made out of Hastelloy C-276, provides precise and stable flow rates and pressures. It is also used for transferring gas or liquid phase to the other parts of the setup and to equilibrate the fluids in the equilibration cell (also made of Hastelloy C-276). The fluid equilibration was performed by retracting the lighter fluid from the top of the cell and injecting it into the bottom of the cell for a given period of time. 2.2.4 Density measurement module - An Anton Paar DMA HPM density meter is used to measure the equilibrium density of phases at elevated pressures and temperatures. The measurement device is a U-shaped Hastelloy C-276 tube that is excited to vibrate at its characteristic frequency, which depends on the density of the fluid. The density meter was located inside the oven and connected to a Rosemount pressure transducer. The operating specifications for DMA HPM are -10 to +200 °C and 0 to 20300 psi. 8
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2.2.5 Temperature control module – This module consists of a resistance temperature device (RTD), sealed in a Hastelloy Sheet (Conax Technologies), a heating jacket (Glas-col, LLC), and a temperature control system. The module uses proportional-integral-derivative (PID) control concept (Watlo Electric Manufacturing, Co.) to maintain an accurate and stable temperature inside the measurement cell. A LabView program, coupled with this control system, was specifically developed to read, plot, and adjust set points for the temperature, the three PID parameters, and to monitor the heat output needed for manual tuning. The controller was finetuned to stabilize the process temperature within ± 0.1 °C. This fine-tuning procedure allows the temperature inside the measurement cell to stay within ± 0.05 °C of the set-point temperature during the measurements for as long as 10 hours. 2.2.6 Data acquisition and analysis module – This includes Leica LAS software, LabVIEW program, and IFT/CA analysis program. These programs were used to capture and analyze images, and to record temperature and pressure. The IFT/CA analysis program uses a wellestablished method, Axisymmetric Drop Shape Analysis (ADSA), to calculate the IFT and CA of drops or bubbles.21 In this technique, the profile of a drop/bubble image is extracted using an edge detection method. The program then fits a theoretical Laplacian curve to the experimental profile extracted from the image assuming an axisymmetric drop/bubble shape. The value of IFT is determined from the best match between these two profiles. A recent improvement in ADSA, Axisymmetric Drop Shape Analysis with No Apex (ADSA-NA), allows IFT/CA calculations without the drop apex information. This means that dynamic contact angles of a drop/bubble can be measured while a protruding needle is attached to it. It has been shown that contact angles can be measured with an accuracy of 0.2° with this method.19 This technique also allows accurate and simultaneous measurement of IFT of drops/bubbles.20 It first calculates IFT using the same 9
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procedure as ADSA, except in this case the drop/bubble image profile does not have an apex. It then uses the final optimum theoretical profile to find the contact angle at the solid surface level. 2.3 Water analysis Carbon dioxide dissolution in water produces carbonic acid, an aqueous solution that becomes more corrosive at high pressures and temperatures.22 The potential effects of corrosion and dissolution of some metal ions of vessels, valves, and connections in the experimental apparatus, prompted an investigation into whether the test fluids become contaminated during the equilibration or measurement periods. Three water samples were analyzed by Inductively Coupled Plasma (ICP) mass spectrometry (1 part per billion accuracy): (1) freshly distilled water from the all-glass water distiller, (2) a certified HPLC grade de-ionized water (VWR international), and (3) the distilled water after being used to make contact angle measurements. The last sample was equilibrated with CO2 by re-circulation through the equilibration cell and associated transfer lines for about one week prior to transferring it to the measurement cell for an additional 10 hours of measurement. The Hastelloy C-276 re-circulation system also contained some stainless steel valves and fittings. Figure 2 shows ICP results for the distilled water, the HPLC grade de-ionized water, the CO2 equilibrated water, and typical chemical compositions of Hastelloy C-276 and stainless steel.23 We find that the distilled water quality (in terms of dissolved ion concentrations) is comparable to the certified HPLC grade de-ionized water purchased from VWR international (Figure 2a). Moreover, the concentrations of ions in the equilibrated water (less than 1 ppm in total) are slightly higher than those in the distilled water (Figure 2b). This was probably caused by the dissolution of metallic components in the system into the CO2 equilibrated water (Figure 2c). However, this insignificant amount of total ion 10
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concentration is not expected to have a significant effect on IFT and CA values reported in this study. 2.4 Experimental procedures 2.4.1 Substrate preparation - The silicon wafers and smooth quartz slides were diced into 19.3 mm × 15.7 mm rectangular pieces compatible with the crystal holder. Each piece of silicon wafer was coated with a layer of Teflon by spin-coating 3% Teflon solution at 1000 rpm according to a procedure by Tavana et al.24 The quartz pieces were used as substrate in the CO2/water/quartz experiments. All the substrates were handled from their edges with clean tweezers to avoid surface contamination. In order to generate strongly water-wet quartz surfaces, the following cleaning procedure was adopted: (1) the quartz substrates were rinsed with Isopropyl alcohol, (2) they were immersed in sulfuric acid solution containing 10% NOCROMIX and sonicated for 30 minutes, (3) the substrates were then soaked inside this solution overnight, (4) they were washed thoroughly with water and boiled in distilled water for about 2 hours, and (5) the quartz pieces were subsequently rinsed and stored inside fresh distilled water. A few minutes before each test, the substrates were dried by absorbing their bulk water with a filter paper and then blow-dried with ultra high pure (UHP) nitrogen. 2.4.2 Substrate surface detection - The contact angle is defined as the angle between a tangent line on a drop/bubble profile and the horizontal plane at the substrate surface. Therefore, precise identification of the solid surface position in a sessile drop or captive bubble image is essential for contact angle measurements. The drop/bubble profiles near the solid surface are sometimes fuzzy and obscure especially for systems with very large (>160°) or very small (
